1. Field of the Invention
The present invention relates to microstrip antennas; and more particularly, to tunable microstrip antennas having bandwidths adjustable by double-stub tuning.
2. Description of Related Art
A microstrip antenna is used for the transmission and reception of electromagnetic energy. As opposed to a conventional wire-based antenna, the microstrip antenna comprises a plurality of generally planar layers including a radiating element, an intermediate dielectric layer, and a ground plane layer. The radiating element is an electrically conductive material imbedded or photoetched on the intermediate layer and is generally exposed to free space. Depending on the characteristics of the transmitted electromagnetic energy desired, the radiating element may be square, rectangular, triangular, or circular and is separated from the ground plane layer. The separation is provided by the intermediate layer, a substrate with a particular dielectric constant, to space the ground plane from the radiating element such that the radiating resonant energy and the corresponding radiation pattern are formed.
A power-driven transmitter and/or receiver network (i.e., transceiver) is generally coupled to the microstrip antenna via a feed point and feed line. Generally, the location of the feed point is selected for optimum matching conditions. When coupled to the transceiver, these three layers contribute to the functions of feed coupling, impedance matching, radiation, and bandwidth shaping.
Microstrip antennas are generally practical for application at frequencies between approximately 1 GHz and 20 GHz. Although no theoretical limit exists, high losses are encountered at frequencies above 20 GHz. Below 1 GHz, wire antennas are more practical because of the large size of the antenna needed.
Microstrip antennas provide advantages such as small size, low weight, low cost, high performance, ease of installation, and aerodynamic profile. Using modern printed circuit techniques, microstrip antennas are mechanically robust when mounted to a rigid surface. They are also versatile elements; they can be designed to produce a wide variety of patterns and polarizations, depending on the mode excited and the particular shape of the radiating element used.
Despite these advantages, a major limitation of a microstrip antenna is its narrow frequency bandwidth. The operating frequency for a microstrip antenna may only be varied from a fraction of a percent to a few percent (approximately 2% to 3%) of its center resonance frequency without severe degradation in performance. The relatively high Q, and hence the narrow bandwidth of the microstrip antenna, is a result of the high dielectric constant of the intermediate substrate layer. However, the high dielectric constant of the intermediate substrate layer allows the desirable physically small size of the microstrip antenna. In essence, the narrow bandwidth results from the radiation impedance infringing capacitance at the edges of the radiating element being much higher than 50 ohms.
One method by which a bandwidth can be increased is by using a matching circuit to drive the antenna. However, the matching circuit takes up additional space on the board, thus effectively adding to the physical size of the antennas and defeating the purpose of the low profile nature of these antennas. The matching network also adds to the loss of the antenna circuit.
In addition to the narrow bandwidth, microstrip antennas have no provision for tuning during and after the manufacturing process. After the feed point is selected, the feed point location, bandwidth, and resonance frequency of the microstrip antenna are fixed.
As discussed above, the use of microstrip or printed circuit techniques to construct antennas has recently emerged as a consequence of the need for increased miniaturization, decreased cost, and improved reliability. However, these microstrip antennas have relatively narrow operational bandwidth which limits tunability of the devices. In general, the antennas should have as wide a bandwidth as possible for various wide band applications.